1. Field of the Invention
The present invention relates to electrochemical gas sensors operated in amperometric mode.
2. Description of Related Art
Electrochemical sensors for toxic gas detection operate in either amperometric or potentiometric mode, i.e. the output current or output voltage is measured as a function of gas concentration respectively. In order for an amperometric sensor to respond to the presence of a toxic gas, the working electrode of the sensor must be held at a potential where the gas can be effectively oxidized or reduced. The electrochemical reaction at the working electrode then generates a current dependent on and usually proportional to the gas concentration.
Amperometric gas sensors have at least a working electrode and a counter electrode. The working electrode provides a suitable catalyst for the electrochemical reaction (oxidation or reduction) of the target gas, while the counter electrode acts to balance out the reaction at the working electrode. If, for example, an oxidation reaction occurs at the working electrode, oxygen will be reduced to form water at the counter electrode. The current flowing through the counter and working electrodes is the same. The majority of amperometric sensors have a reference electrode in addition to a working electrode and a counter electrode. A reference electrode provides a stable, reference potential for the operation of the working electrode.
In order for the sensor to operate properly, the three electrodes are connected to a potentiostatic circuit in such a way that the working electrode potential is controlled vs. the reference electrode potential, and any current produced at the working electrode flows through the counter electrode so that the reference electrode remains un-polarized. Three electrode sensors are well known in the prior art, and are described, for example, in U.S. Pat. Nos. 3,776,832, 3,992,267, 3,824,167, and 3,909,386.
For sensors having only two electrodes, i.e. a working electrode and a counter electrode, the counter electrode also serves as a reference electrode. The catalyst material of this electrode is specially formulated so that polarization occurs to a lesser extent when current passes through the electrode.
The use of a platinum or other precious metal electrode as a reference electrode has greatly simplified gas sensor designs. The potential of a platinum reference electrode is believed to be controlled by the reduction of oxygen to water (1.23 V vs. standard hydrogen electrode under standard conditions, CRC Handbook of Chemistry and Physics, 68th Edition, 1987-1988, CRC Press Inc, Boca Raton, Fla.) and the potential for this electrode is typically found to be about 1.0 V vs the standard hydrogen electrode. Quite a few toxic gases including carbon monoxide (CO), hydrogen sulfide (H2S), sulfur dioxide (SO2), hydrogen (H2), hydrogen cyanide (HCN), phosphine (PH3), silane (SiH4) and ethylene oxide (C2H4O) can be readily oxidized at a potential significantly lower than this potential on an electrode with a suitably active catalytic surface. Some other gases such as nitrogen dioxide (NO2), ozone (O3), chlorine (Cl2) and chlorine dioxide (CO2) can be reduced at a potential significantly higher than this potential.
In order to detect these gases, the working electrode can be set at exactly the same potential as that of the reference electrode. Thus, a three-electrode cell can be operated with a bias of zero between the reference and working electrodes. The external electric circuitry eliminates the large potential difference between the working electrode and the reference electrode and ensures that the sensor is in working condition whether or not a target gas is present.
A two electrode cell can be operated with a zero bias voltage across the cell, either by means of a potentiostat, or more simply by adding a small load resistor (10-100_) between the two electrodes and measuring the voltage drop across the resistor.
Nevertheless, sensors are not always connected to external circuitry. New sensors often need to be stored for many weeks before being installed into an instrument. During the initial 4-12 weeks after manufacture, sensors undergo an important aging process through which their sensitivities drop by 10-30% to more stable values. Keeping electrodes shorted together accelerates this aging process. Gas sensors from instruments in the field are often returned for integrity tests, and some instruments are turned off when not in use.
When a sensor is disconnected from an instrument, its electrodes tend to polarize, i.e. develop a potential difference between them. The final potential difference between the working electrode and the reference or counter electrode depends on the catalytic materials and morphology of the electrodes, nature of the electrolyte, pH and surrounding atmosphere. The environment of the working electrode, which is exposed to the outside atmosphere is different from the reference electrode which is usually within the body of the sensor and thus separated from atmosphere by the electrolyte. Therefore, an open cell voltage between the working and reference electrodes gradually develops. For example, the open cell voltage of a carbon monoxide sensor may be as high as 0.3 volt in relatively clean air.
A large open cell voltage usually results in a high current spike when the sensor is connected to an instrument, and requires a long period of time for the sensor output current to stabilize before the instrument can be used. The time required for a cell to stabilize is referred to as the sensor xe2x80x9cstart-upxe2x80x9d, or xe2x80x9cwarm-upxe2x80x9d time, and is usually somewhere between 15 minutes and more than 24 hours, depending on the type of cell.
In order to keep a sensor in a xe2x80x9cready to workxe2x80x9d condition when the sensor is not connected to any powered circuitry, an electrical shorting link is usually added between working and reference electrodes to minimize polarization of the working electrode. The electrodes and electrolyte in the sensor cell and the external electric means constitute a complete, closed loop which allows current to flow from one electrode to the other. Preferably the electrical means has minimal resistance, so that the two electrode potentials are balanced within a short period of time. Existing shorting means include metal wires, metal springs, custom-made metal containing shorting links and conductive foams. The use of shorting clips is well known in the prior art, for example their use being discussed in U.S. Pat. Nos. 5,906,718, 6,001,240 and 6,074,539. A large current spike, typically at milliampere level, is usually observed upon connecting a shorting link to an open circuit sensor. The peak current is a function of the open cell voltage and the overall resistance in the closed circuitry. The current decays approximately exponentially to about zero when the sensor is surrounded by clean air. Internal and external shorting clips have also been described in U.S. Pat. No. 5,331,310 to combine the counter and reference electrodes in a three electrode electrochemical cell, for use in a two electrode cell circuit. However, in all the prior art examples of shorting clips, the intention was to reduce the resistance between the thus shorted electrodes to essentially zero.
While a shorting link is used in most amperometric gas sensors, manual connection and disconnection of this electric means has been very inconvenient, especially when hundreds or thousands of freshly produced sensors are to be stored and tested. Many gas detection instruments are used by personnel with only minimal training in the art of electrochemical gas sensors and sometimes the removal of the shorting pins is misunderstood or overlooked. Leaving the shorting pin in an instrument can cause a potentially dangerous situation. In addition to the costs associated with extra parts and labor, large current spikes may have a damaging effect on sensors themselves. In addition, most instruments must be designed with an electrical shorting means in place when they are un-powered, adding additional cost and complexity to the instruments. For example, U.S. Pat. No. 4,776,203 describes a potentiostat circuit including an electric connection between the sensing electrode and the working electrode. The connection is eliminated when the circuit is powered, with the making and breaking of the connection accomplished with a field effect transistor (FET) having a low resistance when turned off and a very high resistance when turned on. Similar electrode shorting devices associated with potentiostat circuits are disclosed in European Patent Application No. 220,896 and U.S. Pat. No. 4,776,203. Instead of using a FET, fixed resistors have also been incorporated into the potentiostat circuit. For example, U.S. Pat. No. 5,446,356 describes a potentiostat circuit for use in biasing a three-electrode electrochemical cell in which a direct electrical connection is made between the working electrode and the reference electrode during normal operation of the sensor. The use of an electrical connection in the potentiostat circuit reduces the zero gas current so that additional compensation circuitry is not required. These patents, however, do not offer any means of reducing start up times needed for a sensor not connected to any circuitry.
It is therefore an object of the invention to provide a shorting link for a gas sensor which is always in place, and which requires no adjustment or removal by personnel using the sensor.
It is a further object of the invention to provide a shorting link which reduces start-up time for new sensors.
To achieve these and other objects, the invention is directed to an amperometric electrochemical gas sensor comprising a permanent electrical resistance means disposed between the working electrode of the cell and an the reference electrode. In the case of a two electrode cell, the resistance means is disposed between the working electrode and the counter electrode, since the counter electrode also serves as a reference electrode. The electrical resistance of the permanent electrical resistance means is between about 10 xcexa9 and 200 kxcexa9. This invention thus embodies incorporation of a permanent electrical pathway between the working electrode and at least one other electrode in the cell. The electrode shorting link is an electron-conductive medium, but has a finite resistance to current flow. Because the link is a permanent part of the sensor, it allows electrode potentials to be balanced within a relatively short period of time, but its resistance value is selected such that its presence does not affect the sensor response significantly.